Air bags are increasingly being installed in motor vehicles and have proven to reduce greatly the risk of injury and death to the protected vehicle occupants. The technology of air bags has been the subject of extensive research over many years. One aspect of that research has involved the material of the air bag.
The air bags in current use are usually made of a coated woven fabric. The fabric provides the strength required to endure the large forces exerted on the material upon inflation and upon impact by the vehicle occupant. The coating is present to seal the fabric against gas leakage. The manufacture of the air bag involves sewing two pieces of the fabric material together and also sewing reinforcement and inflation control elements onto the basic bag or envelope.
Coated fabric air bags have three disadvantages. One is that the coated fabric is relatively thick, usually about 400 micrometers. Accordingly, the enclosure in the vehicle into which the air bag is folded in readiness for deployment is of a relatively large size. A second disadvantage is the complexity of the manufacturing process, which makes the air bag expensive to make. A third disadvantage is that the air bag is opaque, so the driver cannot see ahead when the bag is deployed.
An obvious candidate for the bag material, of course, is a polymeric film. Thermoplastic polymeric films can be vacuum-formed to give the air bag a desired shape and can be fusion-bonded, which is a more economical manufacturing technique than sewing. Many suitable polymeric films are transparent, so the driver's view ahead would be preserved. Thus, a polymeric film air bag would overcome the above-mentioned disadvantages.
Many polymeric films, especially uniaxially and biaxially stretched films, have sufficient tensile and rupture strengths for air bags, but they do not have adequate notch tear strength (resistance to tearing at a notch). On the other hand, polymeric films, especially stretched polymeric films, have good edge tear strength (resistance to tearing at an edge with no notch). Because of an insufficient notch tear strength, ordinary polymeric films (unstretched or stretched) are not satisfactory for air bags.